1. Field of the Invention
The present invention relates to a compact, light-weight, imaging lens suitable for digital imaging apparatuses equipped with a solid-state image sensing device such as a charged coupled device (CCD) and a complementary metal oxide semiconductor (CMOS). The present invention further relates to a camera module equipped with the imaging lens and an imaging apparatus equipped with the camera module.
2. Description of the Related Art
In recent years, portable digital imaging apparatuses such as cellular telephones and digital cameras have become widely prevalent. Accompanying demand for more compact digital imaging apparatuses is demand for the imaging lens equipped on such digital imaging apparatuses to be further reduced in size. To satisfy such demands, compact lenses have been proposed such as that disclosed in Japanese Patent Application Laid-Open Publication No. 2007-298572.
The imaging lens disclosed in Japanese Patent Application Laid-Open Publication No. 2007-298572 includes, sequentially from an object, an aperture stop, a positive first lens, a negative second lens cemented to the first lens, a third lens having a concave surface facing toward the object, a fourth lens having a concave surface facing toward the object, and a fifth lens having a convex surface facing toward the object. By having this configuration, the imaging lens is a compact, bright lens system having a short overall length, is suitable for application to a compact imaging apparatus such as a cellular telephone, and capable of favorable correction of various types of aberration.
Cameras equipped on cellular telephones are now capable of not only capturing still images, but also moving images. Conventional compact imaging lenses such as that disclosed in Japanese Patent Application Laid-Open Publication No. 2007-298572 realize, at most, brightness on the order of F2.8. If a cellular telephone equipped with such an imaging lens is used in a dark place to capture a still image, no particular problem arises since a light-emitting device capable of instantaneously emitting a flash of bright light like that of a strobe is provided. However, if a moving image is to be captured, the light-emitting device must be operated continuously since bright light is continuously required during operation of the camera. Longer operation of the light-emitting device increases power consumption and also rapidly depletes the battery, which is a significant problem particularly with compact imaging apparatuses on cellular telephones having small batteries.
One approach to address the problem of power consumption is to increase the aperture size, whereby images can be captured even in dimly lit places, without use of a flash. However, implementing larger apertures on conventional compact imaging lenses such as that disclosed in Japanese Patent Application Laid-Open Publication No. 2007-298572 makes effective correction of aberration difficult. In particular, with the lens disclosed in Japanese Patent Application Laid-Open Publication No. 2007-298572, since the aperture stop is disposed nearest the object, if the aperture size is increased, the correction of spherical aberration, chromatic difference of magnification, etc. becomes difficult. In addition, it becomes difficult to maintain the compact size of the lens.